Acoustic logging tools utilize an acoustically coupled piezoelectric transducer to convert acoustic waves into output electrical signals. A typical acoustic logging tool incorporates an acoustic transmitter which propagates a pulse into the adjacent formation. A return pulse is received at the device. An input pulse is observed at each acoustic receiver. Better data interpretation can be obtained if there are multiple acoustic receivers. In a typical device there are N acoustic receivers, and they output data collectively indicating more subtle relationships. One subtle relationship is in the relative phase between the various receivers. The delay time of the propagated wave front in arriving at the different receivers is also important. A multitude of data is made available by such a device.
The data burst occurs in about 500 to 1,500 microseconds after the pulse is transmitted. Simultaneous data reception and telemetry is especially difficulty over a monocable. A monocable is a cable for supporting a downhole logging tool in a sonde wherein two conductors are included in the cable. While one function as ground, power is delivered on the other conductor to be transmitted from the surface to the sonde. Additionally, this pair of conductors is used to transmit data from the tool to the surface. This two conductor system provides a somewhat limited band width for data transmission, and it is not possible to crowd full data through the monocable in real time from N acoustic receivers.
One data transfer procedure has accommodated this limited data band width in the past is use of multiple firings of the transmitter, each firing dedicated to a single acoustic receiver. Thus, four separately created acoustic pulses would be propagated into the adjacent formations, and each of the four acoustic receivers would be sequentially operated to provide an output signal. This typically occurs as the tool is in transit up the borehole. There is an inevitable shift in position of the sonde between pulses. This movement makes it somewhat difficult to implement various data reduction procedures using N acoustic receivers where there has been a shift in the position of the acoustic transmitter and the respective receivers because each is working with different transmitted pulses. This has created difficulties in data reduction; the data reduction requires shifting to obtain time coincidence of the transmitted pulses. Since the shifted data is not what really happened, such data reduction techniques add to the complexity of interpretation. One important data reduction technique is cross correlation of the transmitted pulse and the one received pulse. The present invention enhances cross correlation.
The present apparatus and method enable use of multiple simultaneously operative acoustic receivers supported in acoustic well logging tools. The tool is lowered in a borehole. At a desired depth, the acoustic transmitter is operated to form a pulse. The pulse is transmitted into the adjacent formations and various acoustic signals are observed at N acoustic receivers on the tool. The various received signals are time multiplexed and stored in memory after conversion into a set of digital words. For each of the N acoustic receivers, the received signal can be reconstructed at the time of data reduction and interpretation. Moreover, the data is stored so that it can be subsequently transferred to the surface on a monocable by means of a suitable telemetry transmitter. With the data from N acoustic receiver signals in memory, the data can be removed from memory at a rate which permits it to be transmitted in the narrow width band available in a monocable. As will be understood, broad band transmission to enable high speed transmission of N simultaneously received acoustic signals is obtained only at the price of a more expensive apparatus capable of broad band transmission. The present apparatus enables the time log between acoustic events to be used for data transmission of N acoustic received events.
Many additional objects and advantages of the present apparatus will be more readily apparent on consideration of the device after its detailed description below. Such device and a method of obtaining data are set forth in detail below.